1. Field of the Invention
The invention relates generally to the field of electronic circuits, and more specifically to circuits for transmitting signals over buses in digital data processing systems.
2. Description of the Prior Art
A digital data processing system includes a number of functional units, including one or more processors, memories, and input/output devices such as mass storage devices, video display terminals, printers and telecommunications devices, all interconnected by one or more buses. The buses carry signals representing information among the various units comprising the system, as well as control signals which, inter alia, control the transfer of the information signals.
Generally, a bus is a set of wires to which several functional units may connect in parallel. When a unit transmits a signal over a wire in a bus, the signal may be reflected when it reaches an end of the bus. Reflected signals may interfere with signals that are later transmitted over the bus wire, which may cause signaling errors over the bus. A major problem with signal reflections is that they can corrupt later transmitted signals. Accordingly, a system designer may have to provide a sufficient delay time following a transmission before another transmission can take place to minimize likely interference from signal reflections.
Alternatively, the system designer may be able to configure the bus or the signals transmitted thereover to minimize reflections. For example, the wires of some include resistor networks at each end which assist in reducing reflections. Power for the bus can also be provided through these bus terminator networks.
In addition, the shape of the signal waveform may be adjusted so as to minimize reflections and crosstalk between signals on different bus lines. In particular, a signal's waveform may be relatively square, in which the voltage level of the bus wire changes relatively abruptly between a high and a low voltage level. Such a signal waveform permits fast signaling, but it also is most likely to cause signal reflections and crosstalk.
On the other hand, if the signals are "trapezoidal" in which voltage levels of the signals change between the high and low levels at a less abrupt, but still rapid, rate, the likelihood of reflections can be minimized. The transmitters which produce such signals must be able to control the rate of change of the voltage on the wires, that is, the "slew rate", within selected limits. This problem is compounded since, in most systems, the buses must be able to handle widely varying numbers of units connected thereto, which, in turn, results in widely varying capacitive load conditions which can vary the slew rate of the signals transmitted over the bus wires.
Currently, transmitters capable of producing trapezoidal signal waveforms are implemented using bipolar transistor devices in combination with discrete resistors and other components whose electrical values can be controlled to a high level of accuracy. Most of the circuits comprising the functional units of digital data processing systems are implemented using MOSFET (metal-oxide-semiconductor field effect transistor) devices, and the bipolar transmitter circuits are implemented separate and apart from the other components comprising the units, which take up a considerable amount of space on the printed circuit boards on which the circuit elements comprising the units are mounted. In addition, the bipolar transmitters, because they are discrete, bipolar and separate from the other devices, require large amounts of electrical power and provide extra delay in the transmitted signal.